Print PageSpatial distributions and abundances of dominant arthropods in grasslands are highly variable, responding to major fire-grazing drivers that alter habitat and quality and interactions with other species. We investigate the basic hypothesis that habitat heterogeneity arising from fire-grazing interactions promotes arthropod diversity and abundance, and modulates species interactions in a food web. Responses of representative, interacting arthropod taxa from multiple trophic levels at Konza Prairie Biological Station in long-term, watershed-level manipulations of fire frequency – bison grazing treatments were sampled. Grasshoppers (Acrididae) and dipteran parasitoids were sampled at 3-week intervals (2007-2009) on 8 watersheds. Prevalence of dipteran parasitoids and grasshopper fecundity was estimated through dissection of grasshoppers; parasitoids were enumerated and grasshopper fecundity determined by evaluating ovariole development. Spider and associated grasshopper abundances were determined at 9 watersheds in 2008 that varied in fire frequency and grazing using 15 x 15 pitfall trap grids. Leaf loss to grasshopper feeding in response to grasshopper:spider densities was assayed by presenting standardized focal wheat leaves of constant foliar quality in water vials near each pitfall trap location. Area removed was measured to assess herbivory level.
Results/Conclusions
Grasshopper species distributions, abundances and fecundity in response to spider predators and dipteran parasitoids respond to long-term fire and grazing treatments at Konza Prairie. Grasshopper species richness and overall abundance increase with bison grazing and to a lesser extent frequent fire, reflecting increased spatial heterogeneity in habitat attributes as well as interactions with natural enemies. Sampling over 3 years documented that parasitism from nemestrinid and tachinid (Diptera) parasitoids was 3x more abundant in watersheds with bison grazing, significantly reducing grasshopper fecundity in some but not all common grasshopper species. A significant negative relationship between grasshopper and spider abundances at local levels also was observed with respect to fire-grazing interactions, and resulted in differential plant risk to feeding by grasshoppers as densities increased. Results will be discussed with respect to trade-offs as consumers respond to variability in food quality, habitat structure, predation risk and microclimate resulting from fire-grazing interactions.
